How do you create a re-useable HTML fragment in wordpress

If this text was static, then of course you could use custom footer template. So create file footer-my-custom.php and then include it with get_footer('footer', 'my-custom');

If you want to edit this text in wp-admin (or have many editable texts in there) you can use one of these solutions:

1. Custom Fields (good if you use static front page)

If you use static front page, then you can add some custom fields to it and then show these fields in footer on every page. To add these CF only to front page, I would use Advanced Custom Fields plugin.

Then in footer you’ll have to add:

<?php echo get_post_meta( get_option('page_on_front'), '<CUSTOM FIELD NAME>', true ); ?>

2. Options

The other solution (better in this case, I guess) is to define some theme options.

I usually use OptionTree plugin to do this. But it’s not so hard to do this by yourself.